1. Field of the Invention
The present invention relates to a thorium oxide-free and, if desired, a cadmium oxide-free optical glass having improved optical constants n.sub.d and .nu..sub.d. In particular, the invention relates to thorium oxide-free optical glass having optical constants of 1.59-1.91 for n.sub.d and 62-19 for .nu..sub.d.
2. Description of the Prior Art
Various high refractive and low dispersive optical glasses have hitherto been reported, but many of them contain thorium oxide which is poisonous to humans because of its radioactivity. However, when they are free of thorium oxide, the content of a single component such as La.sub.2 O.sub.3 becomes markedly larger and thus the tendency toward devitrification is increased, which makes the mass production of such glasses difficult.
It was successful to obtain a thorium oxide-free and stable optical glass by replacing a part or the entire amount of La.sub.2 O.sub.3 with Gd.sub.2 O.sub.3. That is, because the properties of Gd.sub.2 O.sub.3 are similar to those of La.sub.2 O.sub.3, a part of or the entire amount of La.sub.2 O.sub.3 of an optical glass can be replaced with Gd.sub.2 O.sub.3 without changing the optical properties. Thus, by preventing an increase in the tendency toward devitrification caused by the use of a large amount of La.sub.2 O.sub.3, the optical glass can be both stabilized with respect to devitrification and the vitrification range can be enlarged. It, in fact, becomes possible to obtain optical glasses possessing improved optical constants that are not present in conventional optical glasses.
As a glass composition containing Gd.sub.2 O.sub.3, a B.sub.2 O.sub.3 --La.sub.2 O.sub.3 -- Gd.sub.2 O.sub.3 --Ta.sub.2 O.sub.5 system is already known from the specification of French Pat. No. 1,529,337, and an optical glass possessing a large vitrification range and improved optical constants which are not present in conventional optical glasses can be obtained. However, such a glass is lacking in satisfactory stability to devitrification, and even if the most stable composition disclosed in the above patent is employed, the optical glass is not so stable that it can be produced on an industrial scale.
Furthermore, if the content of B.sub.2 O.sub.3 in the B.sub.2 O.sub.3 --La.sub.2 O.sub.3 -- Gd.sub.2 O.sub.3 --Ta.sub.2 O.sub.5 system glass is high, the glass tends to have phase separation. Accordingly, an optical glass having a refractive index n.sub.d of lower than 1.76 has a strong tendency towards devitrification and it has hitherto been difficult to obtain a high refractive and low dispersive optical glass in the refractive index range.